HOLLYWOOD, Fla. — The first patient was rushed into the emergency room of Memorial Regional Hospital around 3 a.m. on Wednesday, escaping a nursing home that had lost air-conditioning in the muggy days after Hurricane Irma splintered power lines across the state.

Another arrived at 4 a.m. After a third rescue call, around 5 a.m., the hospital’s staff was concerned enough to walk down the street to check the building themselves.

What they found was an oven.

The Rehabilitation Center at Hollywood Hills needed to be evacuated immediately. Rescue units were hurrying its more than 100 residents out. Dozens of hospital workers established a command center outside, giving red wristbands to patients with critical, life-threatening conditions and yellow and green ones to those in better shape.

Checking the nursing home room by room, the hospital staff found three people who were already dead and nearly 40 others who needed red wristbands, many of whom had trouble breathing. The workers rushed them to Memorial’s emergency room, where they were given oxygen. The rest went to other hospitals nearby.

Four were so ill that they died soon after arriving. In the afternoon, the authorities learned that another had died early in the morning, and was initially uncounted because the person had been taken directly to a funeral home.

In all, eight were dead.

“We had no idea the extent of what was going on until we literally sent people room to room to check on people,” said Dr. Randy Katz, the hospital’s chairman of emergency medicine.

Three days after the hurricane had howled through South Florida, some of the most vulnerable people in the state were dying, not of wind, not of floods, but of what seemed to be an electrical failure.

Florida was still staggering to its feet on Wednesday, and millions of people across the Southeast were facing days or weeks without power in temperatures that, in the Fort Lauderdale area, climbed to as high as 92 degrees in recent days. The nursing home appeared to have electricity, but the hurricane had knocked out power in a critical spot: A tree had apparently hit the transformer that powered the cooling system, intensifying the subtropical heat from oppressive to fatal.

State officials, utility executives and the Rehabilitation Center spent Wednesday trading blame over why and how its patients were left to endure such conditions, even though state and federal regulations require nursing home residents to be evacuated if it gets too hot inside.

The Hollywood Police Department opened a criminal investigation into the deaths of the eight residents, who ranged in age from 71 to 99, and investigators from the state attorney general’s office were also involved. Gov. Rick Scott ordered a moratorium on admissions at the nursing home.

By day’s end, the unanswered questions were still outstanding, even as the deaths magnified scrutiny on other facilities for the old and disabled.

More than three million customers in Florida still lacked power Wednesday, including roughly 160 nursing homes, according to the state’s tracking system. After generators fizzled at the Krystal Bay Nursing and Rehabilitation Center, in North Miami Beach, 79 people were evacuated as a precaution.

“I am going to aggressively demand answers on how this tragic event took place,” Mr. Scott said in a statement. “Although the details of these reported deaths are still under investigation, this situation is unfathomable. Every facility that is charged with caring for patients must take every action and precaution to keep their patients safe — especially patients that are in poor health.”

Dr. Katz said Memorial’s emergency room had been busy for days treating chronically ill patients who were not coping well with the loss of electricity; some were having trouble breathing in the heat, while others needed access to dialysis. At least one came in from the Rehabilitation Center on Tuesday.

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The Rehabilitation Center at Hollywood Hills, which was evacuated on Wednesday. Eight of its residents died.CreditJason Henry for The New York Times

But not until Wednesday morning was there any hint that others there might be in trouble.

“I don’t know how many more I’m going to get,” said Craig T. Mallak, the chief medical examiner for Broward County, referring to the rising death toll, in an interview. “These are really sick people.”

The home’s administrator, Jorge Carballo, said in a statement that the transformer connected to the air-conditioning system had experienced a “prolonged power failure,” prompting the staff to contact Florida Power & Light. While waiting for a fix, he said, they set up mobile cooling units and fans and tried to make sure residents were hydrated and comfortable.

“We are devastated by these losses,” he said. “We are fully cooperating with all authorities and regulators to assess what went wrong.”

He did not say whether the home had considered evacuating its residents sooner.

Mr. Scott said that the Rehabilitation Center was responsible for the safety of its patients, and that state health officials had told the home’s administrators to call 911 if they believed patients’ health was at risk.

One relative who visited on Tuesday afternoon said she had been so alarmed by the conditions inside that she herself called Florida Power & Light four times. The relative, Eli Pina, said the power company told her that help was on the way. But none came.

“It felt like 110 degrees,” said Ms. Pina, whose 96-year-old mother, Mirelle Pina, was evacuated from the nursing home on Wednesday. “I think it’s the fault of FPL,” she added. “They said they were going to come but they didn’t.”

In an interview with the local ABC station, Dave Long, who worked for an air-conditioning company that serviced the nursing home, said he had been asking Florida Power & Light since Monday to fix a fuse in the system that had “popped” out because of damage from the hurricane.

“We’ve been calling and calling,” Mr. Long said. “I can’t do anything until we get that fuse popped back in.”

Rob Gould, a spokesman for the power company, said at a news conference Wednesday that the company met in March with Broward County officials to discuss hurricane preparations, but that the officials had not flagged the nursing home as “top-tier” critical infrastructure that would need power first. Memorial Regional Hospital, where many residents were taken, was in the top tier.

Broward County officials, though, said in a statement that they had relied on a Florida Power & Light document saying that nursing homes were “non-critical, but play a decisive role in community recovery,” suggesting they were considered a high priority for restoration but not the highest. On Tuesday morning, after the nursing home reported that the air-conditioning was out, county officials asked the utility to make it, along with other nursing homes, a higher priority, the officials said.

The utility “said there were too many to escalate all of them,” Barbara Sharief, the Broward County mayor, said in an interview.

Kristen Knapp, a spokeswoman for the Florida Health Care Association, an advocate for nursing homes, said she was encouraging other facilities to “go ahead and think about moving” residents if they did not think they could keep them safe from the heat.

Florida requires nursing homes to ensure emergency power in a disaster as well as food, water, staffing and 72 hours of supplies. A new federal rule, which takes effect in November, adds that the alternative source of energy must be capable of maintaining safe temperatures.

In general, nursing homes are required to keep temperatures between 71 and 81 degrees, according to the Florida Agency for Health Care Administration. That rule applies to nursing homes certified for the first time after October 1990. However, facilities certified before that time “still must maintain safe and comfortable temperature levels,” the agency’s guidance says.

The causes of death had not been determined Wednesday. Medical professionals said there could be other reasons besides intense heat. Portable generators, as well as other appliances, can cause fatal carbon monoxide poisoning if used indoors.

“It is reasonable to suspect,” said Dr. Beau Briese, an emergency physician at Houston Methodist Hospital who has treated many cases of carbon monoxide poisoning.

One of those who died on Wednesday, Carolyn Jo Eatherly, 78, was living at Rehabilitation Center because of Alzheimer’s she developed many years ago, a close friend, Linda Carol Horton, 65, said Wednesday.

Carolyn Jo Eatherly, left, with her friend Linda Horton in a photo provided by Ms. Horton. Ms. Eatherly, a resident of the Hollywood nursing home, died Wednesday.

As Ms. Eatherly’s dementia progressed, Ms. Horton took her in for as long as she could. But about 10 years ago, Ms. Eatherly had to go into nursing care. Ms. Horton took care of her friend’s four cats until they died.

She hated thinking of Ms. Eatherly helpless in the overwhelming heat.

“I’m really saddened at what happened,” she said.

The 152-bed nursing home was acquired in 2015 by Larkin Community Hospital, a growing Miami-area network that includes hospitals, nursing homes and assisted living facilities.

Florida officials had cited a deficiency related to the building’s generator as recently as February 2016. An inspection called for backup power systems to be “installed, tested and maintained” by March 2016, records show.

While praising the nursing home for above-average staffing, Medicare assigned it an overall “below average” rating, with two of five stars. A health inspection report dated from March raises issues with housekeeping, food service and resident cleanliness, but not with the heating or cooling system.

Dr. Jack Michel, the health-care network’s current chairman, did not respond to requests for comment. Dr. Michel and Larkin Community were among defendants who paid $15.4 million in 2006 to settle federal and state civil claims that the hospital paid kickbacks to doctors in exchange for patient admissions.

Elsewhere in Florida, the grim work of clearing debris and identifying people who had died during the storm was continuing. President Trump planned to visit the Naples area on Thursday.

Besides the nursing home deaths, at least 14 deaths in Florida have been tied to the storm and its aftermath, with six more in South Carolina and Georgia. Across the Caribbean, 38 had died.

At least eight died in the Florida Keys, and authorities feared that many more had drowned as they tried to ride out the storm in their boats. One man died of a stroke while emergency services were unavailable and the hospital was closed.

Among the dead from the Hollywood center was Gail Nova, 71, who had worked as an X-ray and mammography technician before her own health declined.

Her son, Jeffrey Nova, 48, said they had chosen the Rehabilitation Center for its round-the-clock skilled nursing care and proximity to the hospital.

“People died under circumstances where it could have been prevented,” he said. “I want accountability. I think that’s something everyone will want.”